使用Java 8 Optional类

从 Java 8 引入的一个很有趣的特性是 Optional 类。Optional 类主要解决的问题是空指针异常（NullPointerException）。本质上是一个包装类，包装了可能为null的对象，并提供一下便利的工具方法。

empty()/of(T value)/ofNullable(T value) 的区别

• empty 方法生成了一个空的optional对象，该optional对象，包裹的对象为null。
• of 方法的入参必须为一个非null 的对象。若入参为null的对象，该方法将会抛出NullPointerException。因为of方法调用了Optional类的私有构造方法，而该构造方法有调用了Object类中的requireNonNull方法，requireNonNull方法在入参为null时抛出异常。
• ofNullable方法入参若为null，则返回optional内部的空对象；若入参不为null，调用of方法构造optional对象。
  代码片段1：

/**
     * Returns an empty {@code Optional} instance.  No value is present for this
     * Optional.
     *
     * @apiNote Though it may be tempting to do so, avoid testing if an object
     * is empty by comparing with {@code ==} against instances returned by
     * {@code Option.empty()}. There is no guarantee that it is a singleton.
     * Instead, use {@link #isPresent()}.
     *
     * @param  Type of the non-existent value
     * @return an empty {@code Optional}
     */
    public static Optional empty() {
        @SuppressWarnings("unchecked")
        Optional t = (Optional) EMPTY;
        return t;
    }

    /**
     * Constructs an instance with the value present.
     *
     * @param value the non-null value to be present
     * @throws NullPointerException if value is null
     */
    private Optional(T value) {
        this.value = Objects.requireNonNull(value);
    }
    /**
     * Returns an {@code Optional} with the specified present non-null value.
     *
     * @param  the class of the value
     * @param value the value to be present, which must be non-null
     * @return an {@code Optional} with the value present
     * @throws NullPointerException if value is null
     */
    public static  Optional of(T value) {
        return new Optional<>(value);
    }

    /**
     * Returns an {@code Optional} describing the specified value, if non-null,
     * otherwise returns an empty {@code Optional}.
     *
     * @param  the class of the value
     * @param value the possibly-null value to describe
     * @return an {@code Optional} with a present value if the specified value
     * is non-null, otherwise an empty {@code Optional}
     */
    public static  Optional ofNullable(T value) {
        return value == null ? empty() : of(value);
    }

代码片段2：

public final class Objects {
......
 public static  T requireNonNull(T obj) {
        if (obj == null)
            throw new NullPointerException();
        return obj;
    }
......
}

orElse(T other)/orElseGet(Supplier other)/ T orElseThrow(Supplier exceptionSupplier)的区别

• 若optional所包装的对象不为null，orElse方法返回optional对象所包裹的对象；若optional对象所包裹的为null，orElse方法则返回该方法的入参。
• 若optional所包装的对象不为null，orElseGet方法返回optional对象所包裹的对象；若optional对象所包裹的为null，orElseGet方法通过入参other构造一个对象。
• 若optional所包装的对象不为null，orElseThrow方法返回optional对象所包裹的对象；若optional对象所包裹的为null，orElseThrow方法通过入参exceptionSupplier抛出一个异常。

    /**
     * Return the value if present, otherwise return {@code other}.
     *
     * @param other the value to be returned if there is no value present, may
     * be null
     * @return the value, if present, otherwise {@code other}
     */
    public T orElse(T other) {
        return value != null ? value : other;
    }

    /**
     * Return the value if present, otherwise invoke {@code other} and return
     * the result of that invocation.
     *
     * @param other a {@code Supplier} whose result is returned if no value
     * is present
     * @return the value if present otherwise the result of {@code other.get()}
     * @throws NullPointerException if value is not present and {@code other} is
     * null
     */
    public T orElseGet(Supplier other) {
        return value != null ? value : other.get();
    }

    /**
     * Return the contained value, if present, otherwise throw an exception
     * to be created by the provided supplier.
     *
     * @apiNote A method reference to the exception constructor with an empty
     * argument list can be used as the supplier. For example,
     * {@code IllegalStateException::new}
     *
     * @param  Type of the exception to be thrown
     * @param exceptionSupplier The supplier which will return the exception to
     * be thrown
     * @return the present value
     * @throws X if there is no value present
     * @throws NullPointerException if no value is present and
     * {@code exceptionSupplier} is null
     */
    public  T orElseThrow(Supplier exceptionSupplier) throws X {
        if (value != null) {
            return value;
        } else {
            throw exceptionSupplier.get();
        }
    }